1. Field of the Invention:
The present invention relates to a spot welding method and a spot welding apparatus for spot-welding a stacked assembly of three or more workpieces.
2. Description of the Related Art:
For manufacturing automotive bodies, for example, it is occasionally customary to spot-weld a stacked assembly of thick high-resistance workpieces made of high tensile strength steel and a thin low-resistance workpiece made of mild steel.
Japanese Laid-Open Patent Publication No. 2006-055898 discloses a spot-welding process for spot-welding such a stacked assembly. According to the disclosed spot-welding process, when an electric current flows through the stacked assembly in an energization sequence, the pressing force applied to the stacked assembly by a first welding tip and a second welding tip is reduced in an initial stage of the energization sequence and then increased in a late stage of the energization sequence, and a current flowing between the first welding tip and the second welding tip is increased in the initial stage and reduced in the late stage. In other words, the pressing force is set to a low level and the current to a high value in the initial stage, and the pressing force is set to a high level and the current to a low value in the late stage.
The high-resistance workpieces, which are positioned in lowest and central regions of the stacked assembly, have a large electric resistance. Therefore, the contact resistance in the vicinity of contacting surfaces of the high-resistance workpieces is also large. Consequently, a large amount of Joule heat is generated in the vicinity of the contacting surfaces. On the other hand, the contact resistance in the vicinity of contacting surfaces of the high-resistance workpiece in the central region and the low-resistance workpiece in a highest region of the stacked assembly is smaller than the contact resistance in the vicinity of the contacting surfaces of the high-resistance workpieces because the resistance of the low-resistance workpiece is smaller.
When the stacked assembly is spot-welded, a melted area is initially developed in the vicinity of the contacting surfaces of the high-resistance workpieces. In some cases, the melted area between the high-resistance workpieces may grow larger before a melted area is developed in the vicinity of the contacting surfaces of the low-resistance workpiece and the high-resistance workpiece.
According to the spot-welding process disclosed in Japanese Laid-Open Patent Publication No. 2006-055898, the pressing force applied to the stacked assembly is reduced in the initial stage of the energization sequence. As a result, a minute gap may possibly be generated between the workpieces. If such a minute gap is generated between the workpieces and the energization sequence is thereafter continued in order to form a melted area in the vicinity of the contacting surfaces of the low-resistance workpiece and the high-resistance workpiece, then the melted area may possibly be scattered, i.e., sputtering may possibly occur, from the gap between the high-resistance workpieces.
If the energization sequence is stopped, however, a sufficiently large melted area and hence a nugget produced in a solid phase from the melted area are not formed between the contacting surfaces of the low-resistance workpiece and the high-resistance workpiece. Accordingly, it is difficult to achieve a desired bonding strength between the low-resistance workpiece and the high-resistance workpiece.
If the pressing force is increased in the initial stage of the energization sequence in order to avoid the above difficulties, then as the contacting areas of the low-resistance workpiece and the high-resistance workpiece becomes larger, resulting in a reduction in the contact resistance in the vicinity of the contacting surfaces. The amount of generated Joule heat is then reduced, making it difficult to cause the melted area and hence the nugget to glow sufficiently large.